The polypropylene fiber is widely used for various applications because the polypropylene fiber is excellent in properties such as chemical resistance and lightweight property, can be readily melt, is excellent in recycling efficiency, can be readily subjected to incineration disposal while generating no harmful gas such as halogen gas even when the polypropylene fiber is incinerated, and the like. However, the heat resistance of the polypropylene fiber is not sufficiently high among synthetic fibers, and therefore improvement in the heat resistance has been demanded.
For example, as a sheet excellent in recycling efficiency and strength, a polyolefin sheet which has been reinforced with a polypropylene fiber is known. In production of the fiber-reinforced sheet, it is necessary to melt polyolefin at a temperature as high as possible for adhesion of a polyolefin sheet base and the polypropylene fiber from the viewpoints of improvement in productivity, adhesiveness between the polypropylene fiber and the polyolefin sheet base, and the like. However, due to poor heat resistance of the polypropylene fiber, the polyolefin cannot be melt at a high temperature to be formed into a sheet at the time of producing the fiber-reinforced sheet. Therefore, the production rate cannot be increased, and moreover, the adhesion between the polypropylene fiber and the polyolefin sheet is insufficient, resulting in reduced productivity, insufficient strength of a fiber-reinforced polyolefin sheet to be obtained, etc.
Moreover, a fabric formed of the polypropylene fiber is used as a filter. Because the filter is sometimes used in a high temperature environment, the improvement in the heat resistance of the polypropylene fiber has been demanded.
As a conventional technology aiming at the improvement in the heat resistance of the polypropylene fiber, a polypropylene fiber having a heat shrinkage rate at 170° C. for 10 minutes of 10% or lower and a melt peak temperature of 178° C. or higher is known. The polypropylene fiber is obtained by melting and molding a homopolypropylene resin having an isotactic pentad fraction of 96% or more and lower than 98.5% and a melt flow rate (230° C., load of 2.16 kg) of 0.1 to 30 g/10 minutes, and then drawing the resultant (see Patent Document 1).
However, the endothermic peak shape of the polypropylene fiber is a broad double shape or a single shape and the crystals thereof are not uniform. Thus, the heat resistance thereof is not yet sufficiently high.
As another conventional technology, a polypropylene fiber having two DSC endothermic peaks at 155 to 170° C. is known which is obtained by subjecting a polypropylene homopolymer having an isotactic index of 90 to 99% to melt spinning or subjecting the polypropylene homopolymer to melt spinning, and then to drawing (see Patent Document 2).
However, in the polypropylene fiber, the endothermic peak at a lower temperature among the two DSC endothermic peaks serves as an index of the heat resistance of the polypropylene fiber, the endothermic peak shape is broad, and the crystals thereof are not uniform. Therefore, the heat resistance thereof is not sufficient.
Moreover, the polypropylene fiber is a widely used synthetic fiber applied to various industrial materials, and the hydrophobicity poses a problem in many applications. For example, in the applications such as paper and a nonwoven fabric, a fiber as a main component has been required to have a high hydrophilicity in many cases. Moreover, a fiber used as a reinforcement material for various matrix materials has been required to have hydrophilicity from the viewpoints of uniform dispersibility in a matrix, adhesion strength to the matrix, and the like. However, because the polypropylene fiber is hydrophobic and has a poor hydrophilicity, the polypropylene fiber, as it is, is hard to apply to paper, a nonwoven fabric, a reinforcement material, etc., which require hydrophilicity.
Thus, technologies aiming at the improvement in the hydrophilicity and water retentivity of the polypropylene fiber have been conventionally proposed. For example, it is known to produce a water absorbing polypropylene fiber by subjecting, to melt spinning, polypropylene in which a water absorbing resin in the form of particles is uniformly dispersed in a resin using polyethylene wax (Patent Document 3). However, in the case of employing the method, polypropylene, to which particles have been added, is subjected to spinning and drawing, which makes it impossible to avoid influences on spinning property and drawing property. Thus, a polypropylene fiber having a sufficient strength cannot be obtained.
Moreover, it is known to produce a polypropylene fiber having irregularities on the surface by irradiating a polypropylene fiber with ionizing radiation, subjecting a polypropylene fiber to embossing and drawing, or subjecting a polypropylene fiber to melt spinning while changing the taking up speed of the polypropylene fiber, and then drawing the resultant (Documents 4 to 6). However, those methods are applied to a polypropylene fiber having a large fineness, i.e., a single fiber fineness as high as 50 to 100,000 denier, and are difficult to apply to a polypropylene fiber having a small fineness, i.e., a single fiber fineness of 10 dtex or smaller, because the fiber is seriously damaged.
In particular, Patent Document 4 describes a technology of obtaining a monofilament having a fineness of 50 to 50,000 denier, and particularly 3,000 to 12,000 denier, by performing irradiation with ionizing radiation before and after drawing. However, when the method is applied to a polypropylene fiber having a single fiber fineness of 10 dtex or lower, and particularly 3 dtex or lower, the strength is reduced, fluffs are frequently generated, unevenness in the form is noticeable, and there arise problems in the process passing property, quality, and grade.
Moreover, a polypropylene fiber having a single fiber strength of 9 cN/dtex or more and a striated rough-surfaced structure formed along the curved fiber surface is known. The polypropylene fiber is produced by drawing a polypropylene undrawn yarn at 125 to 155° C. in a hot air bath (Patent Document 7). However, in the polypropylene fiber, the intervals between the striated rough-surfaced structures that exist on the fiber surface and the height thereof are small. Therefore, the fiber does not have sufficient water retentivity and the compatibility with a matrix is insufficient.
Further, a method of producing a drawn yarn by drawing a polypropylene undrawn yarn in a single step using pressurized saturated steam of 3.0 to 5.0 kg/cm2 (temperature: 133 to 151° C.) is known (Patent Document 8). However, in the polypropylene drawn yarn (polypropylene fiber) obtained by the method, the formation of irregularities on the fiber surface is insufficient, the intervals between the irregularities and the height thereof are small, the fiber does not have sufficient water retentivity, and the compatibility with a matrix is insufficient.
Moreover, a hydraulic product formed by hydration-curing (hereinafter sometimes referred to as “water curing”) of a hydraulic composition containing hydraulic substances such as cement, gypsum, and water-granulated slag, has generally a low strength and is likely to develop cracks at the time of drying shrinkage.
Thus, a reinforcement fiber has been conventionally added to a hydraulic substance such as cement. As the reinforcement fiber for a hydraulic substance, asbestos has been conventionally used. However, asbestos is harmful to a human body and is not desirable in terms of safety and hygiene. Thus, the use thereof is being regulated now.
In recent years, various inorganic fibers and synthetic fibers are used as the reinforcement fiber for hydraulic substances in place of asbestos. As the synthetic fiber, a polypropylene fiber, a polyvinyl alcohol fiber, an acrylic fiber, etc., are mainly used. Of those, the polypropylene fiber is excellent in alkali resistance and impact resistance, is lightweight, and can be subjected to autoclave curing. Therefore, the consumed amount of the polypropylene fiber has been particularly increased in recent years.
It is known that when autoclave curing is performed at a higher temperature at the time of producing a hydraulic product from a hydraulic substance such as cement, the curing can be completed in a short time. In view of the above, when a reinforcement fiber having high heat resistance can be used, curing time is shortened, which saves a curing space. Further, because the turnover of jigs such as a mold is increased, autoclave curing at a higher temperature is advantageous for the production of the hydraulic product. Moreover, it is known that the autoclave curing temperature influences on the dimensional stability of the hydraulic product. When the curing temperature is higher, the dimensional stability of the hydraulic product to be obtained tends to increase.
However, at present, a polypropylene fiber for reinforcing a hydraulic substance which has a high heat resistance such that the fiber is resistant against autoclave curing, high strength, and excellent compatibility with a hydraulic substance such as cement, has not yet been obtained.
For example, Patent Document 1 mentioned above describes using, as a cement reinforcing material, a polypropylene fiber having a heat shrinkage rate at 170° C. for 10 minutes is 10% or lower and having a melt peak temperature of 178° C. or higher, which is obtained in Patent Document 1. However, in the polypropylene fiber obtained in Patent Document 1, the endothermic peak shape is a broad double shape or a broad single shape, the crystals thereof are not uniform, and the heat resistance is not yet sufficiently high as described above. Therefore, the polypropylene fiber of Patent Document 1 is not suitable for autoclave curing at a high temperature, particularly at a temperature exceeding 150° C., and more particularly at a temperature as high as 170° C. or higher. When the polypropylene fiber of Patent Document 1 is subjected to autoclave curing at a high temperature, the reduction in the strength, degradation, etc., of a polypropylene fiber are likely to occur.
The above-mentioned Patent Documents 4 to 6 describe using a polypropylene fiber obtained in these inventions, which has irregularities on the surface, for reinforcing a hydraulic substance. The polypropylene fibers described in Patent Documents 4 to 6 have a large fineness, i.e., a single fiber fineness of 50 to 100,000 denier. Thus, the compatibility of each of the polypropylene fibers with a hydraulic substance is likely to become insufficient, and moreover, in order to uniformly disperse the polypropylene fibers throughout a hydraulic substance for sufficient reinforcement, a large amount (large mass) of the polypropylene fibers needs to be blended. Moreover, when the methods of forming irregularities described in Patent Documents 4 to 6 are applied to a polypropylene fiber having a fine size, i.e., a single fiber fineness of 10 dtex or smaller, the fiber is remarkably damaged. Thus, it is actually difficult to apply the formation methods to the polypropylene fiber having a fine size.
Of those, the polypropylene fiber described in Document 4 has a large fineness and is difficult to exhibit sufficient reinforcement effects to a hydraulic substance.
Moreover, Patent Document 7 describes using a polypropylene fiber having a single fiber strength of 9 cN/dtex or more and a striated rough-surfaced structure formed along the curved fiber surface for reinforcing concrete. However, as described above, because the polypropylene fiber does not have sufficient water retentivity, the compatibility with a hydraulic substance is insufficient.
Further, the above-mentioned Patent Document 8 refers to a reinforcement fiber for cement as one of the applications of the polypropylene drawn yarn obtained in the invention of Patent Document 8. However, as described above, in the polypropylene drawn yarn (polypropylene fiber) obtained by the method of Patent Document 8, the formation of irregularities on the fiber surface is insufficient, the intervals between the irregularities and the height thereof are small, and the polypropylene fiber does not have sufficient water retentivity. Therefore, the compatibility with a hydraulic substance forming a matrix is insufficient.
A rope formed of a fiber has so many applications, for example, land/marine transportation, fishery, agriculture, and construction sites. As a fiber material for a rope formed of a fiber, both a natural fiber and a synthetic fiber are used. In recent years, a rope formed of a synthetic fiber is mainly used. As the rope formed of a synthetic fiber, a nylon fiber, a vinylon fiber, a polyester fiber, a polypropylene fiber, a polyethylene fiber, a polyvinyl chloride fiber, etc., are mentioned. Of those, a rope formed of a polypropylene fiber has advantages in that the rope formed of a polypropylene fiber is excellent in chemical resistance, lightweight property, etc., can be readily melt, is excellent in recycling efficiency, is subjected to incineration disposal while generating no harmful gas such as halogen gas even when the polypropylene fiber is incinerated, and the like. Therefore, various proposals on the rope formed of a polypropylene fiber, a method of producing a polypropylene fiber for use in the rope, etc., have been suggested (see Documents 9 and 10).
However, among synthetic fibers, a heat resistance of the polypropylene fiber is not high. Thus, the improvement in the heat resistance of a rope formed by using the polypropylene fiber has been demanded. This is because, when the rope formed by using the polypropylene fiber is exposed to a high temperature or subjected to frictional heat at the time of rubbing or scratching, the polypropylene fiber forming the rope melts, causing, for example, meltdown of the rope, which results in that physical properties such as strength are likely to decrease and the drawing of the rope under a high temperature is high.
As described above, Patent Document 1 discloses a polypropylene fiber whose heat resistance has been attempted to increase, which has heat shrinkage rate at 170° C. for 10 minutes of 10% or lower and melt peak temperature of 178° C. or higher. The polypropylene fiber is obtained by melting and molding a homopolypropylene resin having an isotactic pentad fraction of 96% or more and lower than 98.5% and a melt flow rate (230° C., load of 2.16 kg) of 0.1 to 30 g/10 minutes, and then drawing the resultant. However, in the polypropylene fiber, the crystals are not uniform and the heat resistance is not yet sufficiently high. Therefore, even when a rope is formed by using the polypropylene fiber, meltdown and reduction in physical properties due to frictional heat or the like are likely to occur. Thus, the drawing at a high temperature is likely to become high.
Moreover, similarly as in ropes formed of other synthetic fibers, it is important also in the rope formed of a polypropylene fiber that, when fibers are twisted, there is no slipping between fibers and between fiber strands and engagements among the fibers and among the fiber strands are firm and in tight twist, in terms of the inhibition of untwist of the fibers or the fiber strands and the improvement in the strength, drawing resistance, wearing resistance, and shape retentivity.
However, with a rope formed of a conventional polypropylene fiber, slipping between polypropylene fibers and slipping between polypropylene fiber strands are high, and it is difficult to sufficiently firmly and tightly twist the fibers.
As a method of reducing slipping between polypropylene fibers and slipping between polypropylene fiber strands, irregularities are formed on the surface of polypropylene fiber and the surface of the polypropylene fiber is roughened. However, in a conventionally known polypropylene fiber on the surface of which irregularities have been formed and a polypropylene fiber whose surface has been roughened, the irregularities (surface roughened) are insufficient and the formation of irregularities is regulated. Even when a rope is formed using the polypropylene fibers, it is difficult to tightly and firmly twist the polypropylene fibers (polypropylene yarn and strand). Thus, a rope formed of the polypropylene fiber which is excellent in strength, drawing resistance, wearing resistance, shape retentivity, etc., cannot be obtained.
For example, in the polypropylene fibers each having irregularities on the surface for reinforcing a hydraulic substance, which are described in Patent Documents 4 to 6 mentioned previously, the damage thereto is likely to be generated. Thus, such polypropylene fibers are applied to the production of a rope, a rope formed of a polypropylene fiber which is excellent in mechanical property, wearing resistance, shape retentivity, etc., cannot be not obtained.
Moreover, with respect to the polypropylene fiber for reinforcing concrete described in Patent Documents 7 mentioned previously, the intervals between the striated rough-surfaced structures that exist on the fiber surface and the height thereof are small. Therefore, the anti-slip effect between fibers is insufficient, and even when the polypropylene fiber is applied to production of a rope, the fibers are not tightly and firmly twisted. Thus, a rope which is excellent in mechanical properties, wearing resistance, untwisting resistance, shape retentivity, etc., cannot be obtained.
Further, with respect to the polypropylene drawn yarn (polypropylene fiber) described in Patent Document 8 mentioned previously, the formation of irregularities on the fiber surface is insufficient and the intervals between the irregularities and the height thereof are small. Therefore, the anti-slip effect between fibers is insufficient, and even when the polypropylene fiber is applied to production of a rope, the fibers are not tightly and firmly twisted. Thus a rope which is excellent in mechanical properties, wearing resistance, untwisting resistance, shape retentivity, etc., cannot be obtained.
Moreover, the polypropylene fiber is used for the production of a sheet-shaped fiber structure such as a woven or knitted fabric, a nonwoven fabric, a synthetic paper, and a net-like article while taking advantage of the properties such as chemical resistance, lightweight property, easiness of recycling, and non-generation of harmful gas at the time of incineration.
Depending on the intended use of the sheet-shaped fiber structure formed of a polypropylene fiber, heat resistance has been demanded. For example, as described above, in production of a polyolefin sheet reinforced with a fabric formed of a polypropylene fiber, a polyolefin base sheet needs to be melted at a high temperature for adhesion between the polyolefin sheet base and the fabric formed of a polypropylene fiber from the viewpoints of the improvement in productivity and adhesiveness between the fabric formed of a polypropylene fiber and the polyolefin sheet base. However, since the heat resistance of the fabric formed of a polypropylene fiber is insufficient, the polyolefin sheet base cannot be melted at a high temperature, which causes the reduction in productivity, lack of adhesion strength between the fiber formed of a polypropylene fiber and the polyolefin base, etc. Further, also when a sheet-shaped fiber structure formed of a polypropylene fiber is used for a filter, a separator, clothes (in particular, sportswear and the like), etc., the improvement in the heat resistance has been demanded. This is because the sheet-shaped fiber structure formed of a polypropylene fiber is sometimes used under a high temperature environment or a state where friction is generated.
The polypropylene fibers described in Patent Documents 1 and 2, whose heat resistance has been attempted to increase, may be used for the production of a sheet-shaped fiber structure. However, in the polypropylene fibers described in Patent Documents 1 and 2, the crystals are not uniform and the heat resistance is not yet sufficiently high. Thus, a sheet-shaped fiber structure excellent in heat resistance cannot be obtained.
Moreover, a synthetic paper and a nonwoven fabric formed of a polypropylene fiber are used for industrial materials such as a filter and a separator. However, due to poor hydrophobicity, the synthetic paper and the nonwoven fabric formed of a polypropylene fiber, as they are, are difficult to be applied to an aqueous filtration and an alkali secondary battery separator that require high hydrophilicity.
As described above, Patent Document 3 describes a water-absorbing polypropylene fiber obtained by subjecting, to melt spinning, polypropylene in which a water-absorbing resin in the form of particles has been added and dispersed using polyethylene wax. However, the polypropylene fiber has insufficient strength, and thus, sufficient strength cannot be obtained when formed into a sheet-shaped fiber structure such as a woven knitted fabric, a nonwoven fabric, a synthetic paper, and a net-like article.
Moreover, even when the polypropylene fibers, described in Patent Documents 4 to 8, each having irregularities on the surface are used for the production of a sheet-shaped fiber structure, such as a woven or knitted fabric, a nonwoven fabric, a synthetic paper, and a net-like article, a sheet-shaped fiber structure which has high water retentivity and is excellent in strength cannot be obtained. This is because the irregularities (surface roughened) are insufficient, so the formation of the irregularities is limited, and because the strength of a polypropylene fiber itself is low.
To be specific, with respect to the polypropylene fibers (in particular, a polypropylene fiber having a small fineness, i.e., a single fiber fineness of 10 dtex or lower) obtained by the methods of forming irregularities described in Patent Documents 4 to 6, particularly in Patent Document 4, the generation of damage is remarkable. Therefore, even when a sheet-shaped fiber structure is formed using the polypropylene fiber, a sheet-shaped fiber structure excellent in strength cannot be obtained.
Moreover, with respect to the polypropylene fiber described in Patent Document 7, the intervals between the striated rough-surfaced structures that exist on the fiber surface and the height thereof are small. Thus, even when the polypropylene fiber is used, a sheet-shaped fiber structure excellent in water retentivity cannot be obtained. Further, with respect to the polypropylene fiber described in Patent Document 8, the formation of the irregularities on the fiber surface is insufficient and the intervals between the irregularities and the height thereof are small. Thus, even when the polypropylene fiber is used, a sheet-shaped fiber structure excellent in water retentivity cannot be obtained.
Further, as one of the applications of a polypropylene fiber, the use thereof as a reinforcement fiber for organic polymers is mentioned. As a specific example thereof, the above-mentioned polypropylene fiber-reinforced polyolefin sheet is mentioned. However, because the heat resistance of the polypropylene fiber is insufficient, the productivity decreases and the adhesion strength between the polypropylene fiber and the polyolefin base is insufficient as described above.
Moreover, when the polypropylene fiber is used as a reinforcement fiber for organic polymers other than polyolefin to produce a composite material containing the polypropylene fiber and an organic polymer and a molded product, a sufficient reinforcement effect is sometimes not acquired due to low heat resistance of the polypropylene fiber and low adhesiveness of the polypropylene fiber to the organic polymer. From those viewpoints, a polypropylene fiber excellent in heat resistance and, moreover, is excellent in adhesiveness to an organic polymer has been demanded. However, the heat resistance of each of the polypropylene fibers described in Patent Documents 1 and 2 is not yet sufficiently high. Thus, the polypropylene fibers described in Patent Documents 1 and 2 are not necessarily effective as the reinforcement fiber for organic polymers.
Further, with respect to the polypropylene fibers having irregularities on the surface described in Patent Documents 4 to 8, the irregularities on the surface (surface roughened) are insufficient, the formation of irregularities is regulated, and the strength is insufficient. Therefore, even when these polypropylene fibers are applied to a reinforcement fiber for organic polymers, it is impossible to obtain a composite material containing an organic polymer and the polypropylene fiber, a molded product, etc., which are excellent in strength and the like due to insufficient adhesion with organic polymers.
Patent Document 1: JP 2002-302825 A
Patent Document 2: JP 2001-20132 A
Patent Document 3: JP 04-41710 A
Patent Document 4: JP 61-26510 B
Patent Document 5: JP 56-9268 A
Patent Document 6: JP 61-301 B
Patent Document 7: JP 2003-293216 A
Patent Document 8: JP 3130288 B
Patent Document 9: JP 07-90785 A
Patent Document 10: JP 2002-20926 A
Non Patent Document 1: “Macromolecules”, vol. 6, 1973, p925
Non Patent Document 2: “Macromolecules”, vol. 8, 1975, p687